1. Field of the Invention
This invention relates to dual-wavelength sensors. More particularly, this invention relates to dual mode millimeter wave and optical sensors employing a common aperture.
2. Description of the Background Art
Dual mode (millimeter wave (MMW) and optical) sensors have traditionally used curved reflectors to collect MMW energy and optical energy and focus such energies on appropriate detectors. In some embodiments, separate apertures for each wavelength band are employed. However, in aperture-limited applications such as missiles or covert sensors, a common aperture is usually employed so as to allow each sensor to collect the maximum incoming energy.
Representative patents include U.S. Pat. No. 4,652,885 entitled "Dual Mode Antenna for Millimeter Wave and Infrared Radiation" and U.S. Pat. No. 4,636,797 entitled "Dual Mode Dichroic Antenna/Aperture". In both of these patents, there is disclosed a dual mode antenna that allows both millimeter wave and infrared radiation to enter a single aperture and propagate through a common transmission device to a point where the respective energies are divided to follow separate paths for subsequent processing. Both patents teach the use of cassegrainian optics for directing the incoming millimeter and infrared energy into the common transmission device in the form of a waveguide. Other representative patents of lesser relevance to the subject invention include U.S. Pat. No. 4,866,454 entitled "Multi-Spectral Imaging System" and U.S. Pat. No. 4,282,527 entitled "Multi-Spectral Detection System with Common Collecting Means". The disclosure of each of the above-referenced patents are hereby incorporated by reference herein.
Presently, there exists a need for increasingly robust gun-launched precision munitions applied to indirect fire against point targets. In order to satisfy such need, autonomous fire-and-forget and man-in-the-loop terminal guidance sensors must be integrated into a single munition. It has been known that an active millimeter wave (MMW) sensor provides the autonomous fire-and-forget terminal guidance capability in adverse weather. Complementarily, a semi-active laser (SAL) that tracks a laser-designated target allows man-in-the-loop terminal guidance capabilities. Combined synergistically, MMW and SAL sensors provide precision guidance to achieve a high hit probability with minimal collateral damage against a broad spectrum of targets and engagements in a variety of battlefield conditions. Unfortunately, successful achievement necessitates integrating the dual sensors (MMW and SAL) with conflicting design constraints into a single compact package with a common aperture.
More specifically, the challenge associated with a common aperture SAL/MMW sensor is the collimation and detection of two widely diverse operating wavelengths on a non-interfering basis. Sensor elements that are used by both SAL and MMW must either be broad-band to encompass both operating wavelengths or have selective coatings to pass one wavelength while reflecting the other. Conversely, sensor elements that are utilized by either SAL or by MMW, but not both, must not significantly interfere with the performance of the other. Accordingly, any acceptable solution must be innovative and employ the appropriate materials to achieve good dual mode performance, relative simplicity and low cost.
Therefore, it is an object of this invention to provide an improvement which overcomes the aforementioned inadequacies of the prior art devices and provides an improvement which is a significant contribution to the advancement of the dual mode common aperture sensor art.
Another object of this invention is to provide a common aperture, dual mode semi-active laser (SAL) and millimeter wave (MMW) sensor apparatus and method, comprising (1) shared elements including a transreflector, a fresnel lens, and a separator, with the transreflector being configured to allow transmission of SAL energy therethrough, with the fresnel lens being configured to pass MMW energy therethrough and to focus SAL energy passing therethrough, and with the separator being configured to separate SAL energy from MMW energy; (2) a field lens positioned in alignment with the SAL energy flowing from the separator for focusing the SAL energy onto a detector; and (3) a twist reflector positioned in alignment with the transreflector, the twist reflector being configured to rotate the polarization of the MMW energy such that, for received MMW energy, the twist reflector reflects and rotates the received MMW energy toward the transreflector whereupon it is reflected therefrom onto a feed and comparator and, for transmitted MMW energy from the feed and comparator, the transmitted MMW is reflected from the transreflector toward the twist reflector whereupon polarization of the transmitted MMW energy is rotated and reflected therefrom to then pass through the transreflector, such that the sensor of the invention is operable in a SAL mode and a MMW mode.
The foregoing has outlined some of the pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.